In advancing the design of communications networks, a trend is developing toward utilizing existing AC power lines as a communications medium. There is a significant advantage to the use of AC power lines for communication purposes. The installation cost for adding dedicated communication wiring and hardware to existing buildings and structures can easily exceed the cost of the system hardware itself. This provides a strong motivation to use existing power lines for communication purposes, as well as for power distribution.
In the past, there has been limited acceptance of power line communication (PLC) systems. Since power lines were never originally intended to provide a medium for communications, there are numerous problems and impairments which must be overcome in order to develop a reliable communication system utilizing existing power lines. Very often, these problems have led to inconsistent and unreliable performance in previous PLC systems.
One of the major problems which must be overcome by a PLC system is the ability to deal with the numerous noise sources and significant signal attenuation which arise in the power line environment. It is well known that most electronic and electrical products (e.g., household appliances) generate significant electrical noise which is eventually coupled back across the power lines. Attenuation of the communications signal results from the frequently lengthy connections and varying impedances seen at the receptacle points. Practitioners in the communications art have long recognized that the combination of signal attenuation plus high noise levels poses an especially difficult communication problem.
As will be seen, the present invention provides a relatively simple, low-cost drive amplifier adapted for power line communication systems. The invented amplifier circuit provides a low output impedance during transmit mode, and a high impedance looking back into the amplifier while operating in a receive mode. Special control circuitry is included to permit rapid switching between transmit and receive modes. Numerous other features desirable of a power line communications amplifier are also incorporated.